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March 2006 ? https://www.wendangku.net/doc/f711871374.html,
T
his article uses Gas Chroma-
tography-Mass Spectroscopy (GC/MS), Thermogravimetric Analysis
(TGA), and X-ray Photoelectron Spectroscopy
(XPS) to characterize the relative thermal properties of a novel high temperature (HT) resistant OSP coating. The GC work performed in this study shows the key components in the HT OSP coating that affect solderability. The GC work also shows the alkyl benzimidazole-HT used in HT OSP has the lowest volatility. The TGA data also illustrates that the HT OSP coatings have a higher decomposition temperature compared to existing industry standard OSP coatings. The XPS shows that HT OSP has only about 1% increase of oxygen content after fi ve lead-free refl ow cycles. These improvements are then related to the industry’s challenges of lead free soldering.
OSP coatings have been used in the PWB industry for many years. They are based on the reaction of azole derivatives with transition metals, such as copper and zinc, to form thin organometallic polymeric coatings. Extensive research [1,2,3] has been conducted to reveal the corrosion inhibition mechanism of azole compounds on metals. G.P . Brown [3] successfully synthesized organometallic polymers of benzimidazole with copper (II), zinc (II) and other transition metals and characterized the unusually high thermal stability of poly-(benzimidazole-zinc) by using TGA. TGA data from his work shows that poly-(benzimidazole-zinc) has a decomposition temperature as high as 400°C under ambient atmosphere and as high as 500°C under nitrogen atmosphere, while the decomposition temperature of poly-(benzimidazole-copper) is only 250°C. The recently developed novel HT OSP is based on the poly-(benzimidazole-zinc) chemistry, which offers superior thermal stability.
OSP is mainly composed of organometallic polymer with small molecules such as fatty acids and azole derivatives entrained in the coating during deposition. The organo-metallic polymer provides the necessary corrosion resistance, adhesion to copper
and surface hardness. T o withstand the lead-free assembly processes, the decomposition temperature of organometallic polymer has to be higher than the melting point of lead-free solders. Otherwise, the OSP would be degraded after passing through lead-free assembly processes. The decomposition temperature of OSP largely depends on the nature of the organometallic polymer. Another key factor that affects protection of copper from oxidation is the volatility of azoles such as benzimidazoles and phenyl-imidazoles. During the lead-free refl ow process, the small molecules in the OSP coatings evaporate, which causes some loss of protection of copper from oxidation. T o scientifi cally characterize the thermal resistance of OSPs, GC/MS, TGA, and XPS were used.
e XPeRiMental GC/MS Study
Test samples of OSP materials were obtained by scraping approximately 0.74–0.79 mg of the OSP coatings from copper
By Shenliang Sun, Yung-Herng Yau, John Fudala, Robert Farrell, Chonglun Fan,
Chen Xu, Karl Wengenroth, Michael Cheung and Joseph Abys
Organic solderability preservative (OSP) coatings are among the leading surface ? nish options in lead-free soldering because of their excellent solderability, ease of processing, and low cost.
panels. These test panels were coated respectively with: a) the new HT OSP, b) the industry standard OSP and, c) another commercially available OSP. Neither the panels nor the coating material samples were processed through any assembly reflow processes prior to GC/MS testing. A H/P 6890 GC/MS was used in syringeless injection mode. The Syringeless Injector gives the capability to perform thermal desorption of solid samples directly within the injection port of a gas chromatograph. The Syringeless Injector transfers the sample, contained within a glass sample vial, into the injection port. Carrier gas continuously sweeps volatile compounds from the sample into a capillary column for collection and separation. Positioning the sample in close proximity to the capillary column results in efficient and reproducible thermal desorption. After sufficient sample has been transferred the sample is expelled up and out of the injection port, where it can be removed or reinjected. The GC column used is a Restek RT-1 GC (0.25 mm ID x 30 m, 1.0 μm film). The GC oven temperature program applied was 35°C to 325°C at 15°C/minute with 2 min. hold at 35°C. Thermal desorption was at 250°C for 2 minute splitless. Volatiles are continuously transferred into the column by helium flow, where they are trapped by using liquid nitrogen cooling of the head of the column. After the heating period, the sample vial is forced out of the injection port with gas pressure. Identifications were made by mass spectra of the whole mass range (from 10-700 daltons). Retention times were determined for each compound. Thermogravimetric Analysis (TGA) Study
T est samples of OSP material were obtained by scraping approximately 17.0 mg of the coating material from copper panels. The panels were also coated with the new HT OSP; the industry standard OSP, and another commercially available OSP product. Neither the panels nor the coating material samples were processed through any assembly reflow processes prior to TGA testing. The TGA was run in nitrogen atmosphere with TA Instrument 2950TA. The working temperature holds at room temperature for 15 minutes, then elevates to 700°C at the rate of 10°C/minute.X-ray Photoelectron
Spectroscopy (XPS) Analysis
X-ray Photoelectron Spectroscopy (XPS) also
called Electron Spectroscopy for Chemical
Analysis (ESCA) is a chemical surface
analysis method. XPS measures the chemical
composition of the outermost 10 nm of a
sample. Copper panels were coated with HT
OSP and industry standard OSP, then passed
through five lead-free reflow cycles. The XPS
analysis of HT OSP before and after five
lead-free reflow cycles, and industry standard
OSP after five lead-free reflow cycles were
conducted with VG ESCALAB Mark II.
Through-Hole
Solderability Testing
Through-Hole Solderability was tested
using Solderability T est Vehicles (STVs) (see
Figure 1). A total of ten STV arrays (four
STVs per array) were coated to a thickness
of approximately 0.35 microns. Five arrays
were coated with the HT OSP, and five were
coated with the industry standard OSP,
using conveyorized horizontal processing.
Afterwards, the coated STVs were subjected
to a series of high temperature, lead-free
reflow cycles in a solder paste reflow furnace.
The reflow series consisted of 0, 1, 3, 5 or 7
consecutive reflow cycles per test condition,
all under an air atmosphere. Four STVs
from each coating set were processed per
reflow test condition. Following reflow
conditioning all STVs were processed
through a high temperature, lead-free wave
soldering process. Through-hole solderability
was measured by inspecting each STV and
counting the number of properly filled
through-holes. The acceptance criteria are
that solder fillet must fill to the top or the
knee of the plated through hole and can also
extend to the top of the topside pad.
Solderability by
Wetting Balance
Solderability was also evaluated by wetting
balance testing. The wetting balance
coupons were coated with HT OSP and
treated with up to 7 lead-free reflow cycles,
T peak =262°C. The reflows were conducted
in air by using a BTU TRS combination
IR/convection reflow oven. The wetting
balance tests were conducted per IPC/EIA J-
STD-003A section 4.3.1 4 using a “Robotic
Process Systems” automated wetting balance
tester with EF-8000 flux, a no-clean flux and
SAC 305 alloy solder.
Solder Joint Strength Tests
The solder joint strength was measured by
shear test. T est boards of BGA pads (0.76
mm diameter) were coated with HT OSP
at a thickness of 0.25 and 0.48 microns and
subjected to three lead-free reflow cycles
with peak temperature of 262°C. Solder
balls composed of SAC 305 alloys (0.76 mm
diameter) were soldered onto the pads with
matching solder pastes. The solder ball was
sheared off at 200 μm/second by using a
Dage PC-400 bond tester.
R esults and d iscussion
GC-MS Analysis
The GC-MS is a very useful tool that can
be used to monitor the evaporation behavior
of the organic ingredients in OSP coatings.
Various azole derivatives including imidazoles
and benzimidazoles are used in various
OSP products in the industry. The alkyl
benzimidazole used in the HT OSP, alkyl
benzimidazole used in the STD OSP and
the aryl phenylimidazole used in other OSP
coatings were evaporated during heating in
a GC column. The azole derivatives that co-
polymerize with metals cannot be detected
by GC/MS because the organometallic
polymer does not evaporate. Therefore,
GC/MS only detects the azole compounds
that do not react with metals and other
small molecules. Normally,
the
Figure 1. Through-Hole
Solderability Test Vehicle.
Note: 1196 through holes per
solderability test vehicle (STV).
10 mil holes-Four grids, 100 holes
each grid, square and round pads;
20 mil holes-Four grids, 100 holes
each grid, square and round pads;
30 mil holes-Four grids, 100 holes
each grid, square and round pads.
https://www.wendangku.net/doc/f711871374.html, ? March 200631
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March 2006 ? https://www.wendangku.net/doc/f711871374.html,
molecule that has lower volatility has longer retention time under exactly same heating and flow conditions in a GC column. Figure 2 shows the GC/MS of the HT OSP with a retention time of 20.5 minutes for the HT alkyl benzimidazole. By comparison (GC/MSs not shown here), the standard alkyl benzimidazole and aryl phenylimidazole peaks appear in 19.0, and 19.0 minutes respectively indicating the higher volatility of these compounds.
GC-MS also showed that the HT OSP had the least impurities of the three OSPs. Organic impurities in OSP coatings cause some loss of solderability and discoloration during reflow processing.
Koji Saeki [5] reported that the poly- merization on the surface of OSP should be weaker than that on the bottom because of less density of copper ion. It is believed that unreacted azoles exist in the upper layer of OSP . During reflow processes, more copper ions migrate from the bottom to upper layer. These unreacted azoles in the upper layer will have the chance to react with copper ions and prevent the copper oxidation. The alkyl benzimidazole-HT used in HT OSP has a lower volatility and a better chance to react with copper ions migrating from the bottom layer, thus reducing oxidation during reflow. X-ray Photoelectron Spectroscopy can show copper ion migration from the bottom layer to the upper layer, which will be discussed later.
Thermogravimetric Analysis (TGA)
The TGA measures weight changes in materials with regard to temperature and allows for the effective quantitative analysis of mass changes. In our case, TGA is also an ideal method to reproduce the evaporation and decomposition behavior of OSP coatings during the first lead-free reflow process in nitrogen atmosphere.
TGA shows that the industry standard OSP has a decomposition temperature of 259°C compared to 290°C for HT OSP (Figure 3). Poly-(benzimidazole-zinc) chemistry has a decomposition temperature of 400°C. However, the actual decomposition temperature of HT OSP is not as high as 400°C because copper co-deposits in the HT OSP coatings. The lower value of 259°C for the industry standard OSP is due to poly(benzimidazole-copper) chemistry. Interestingly, the other OSP has two decomposition temperatures, which are 256°C and 356°C.
This
Figure 2. HT OSP before lead-free reflow.
Figure 3. TGA of HT OSP.
Figure 4. XPS of HT OSP after five lead-free reflow cycles.
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March 2006 ? https://www.wendangku.net/doc/f711871374.html,
OSP may contain iron [6] and the stepwise decomposition may possibly be the attribute of poly-(aryl phenylimidazole-iron). TGA results from F . Jian and his associates show that poly-(imidazole-iron) also has two decomposition temperatures of 216°C and 378°C [7].
X-ray Photoelectron Spectroscopy Analysis
X-ray Photoelectron Spectroscopy utilizes photo-ionization and energy-dispersive analysis of the emitted photoelectrons to study the composition and electronic state of the surface region of a sample. The binding energy peak of oxygen (1s), copper (2p) and zinc (2p) in XPS appear at 532–534 eV, 932–934 eV, and 1022 eV respectively. This technique can provide a quantitative analysis of the surface compositions of the outermost 10 nm of a sample. By this analysis HT OSP has 5.02% of oxygen and 0.24 % of zinc before lead-free reflow process. Figure 4 shows that HT OSP has 6.2% of oxygen and 0.22% of zinc after five times lead-free reflow cycles. Copper content increases from 0.60% to 1.73% after five lead-free reflow cycles. The possible reason of copper ion increase is the copper ion migration from bottom to the top layer during reflow cycles.
E. K. Chang, et al, [8] also conducted surface analysis of the industry standard OSP by using X-ray Photoelectron Spectroscopy. The oxygen content was 5.0% without passing through any reflow process, which
then increases to 9.1% and 11.0% after one and three conventional SnPb reflow cycles in air respectively. It was also reported that the oxygen content increases to 6.5% after one SnPb reflow cycle in nitrogen. The oxygen content increased to 12.5% after five lead-free reflow cycles. Therefore, the industry standard OSP has a 7.5% increase in oxygen content after five lead-free reflow cycles, which is larger than HT OSP .
Solderability is largely determined by the degree of oxidation on the joining surfaces and the aggressiveness of the applied flux. Therefore, the oxygen content determined by XPS is a very effective indicator of the thermal resistance of OSP . HT OSP exhibits excellent thermal resistance compared to the industry standard OSP .
Discoloration tests show that HT OSP has little discoloration after five lead-free reflow cycles, while industry standard OSP has significant discoloration after five cycles. The discoloration results are consistent with the XPS results.
Solderability Tests
Solderability tests show that the through-hole solderability of the HT OSP is superior to that of existing industry standard after multiple lead-free reflow cycles. This is consistent with the good thermal resistance of HT OSP . As the number of reflows increases, time to zero (T o ) gradually increases and the maximum wetting force decreases slightly. However, excellent solderability is
maintained by the HT OSP through seven lead-free reflow cycles. Shear tests show that the shear force increases gradually and reaches the maximum at 25N. Since the shear strength depends on the cross sectional area at which the shear takes place, the results would vary with the shape of the solder ball and the clearance between the shear and pad. The shear force is independent of OSP thickness as long as the copper surface has enough protection from oxidation.
Conclusions
? Alkyl benzimidazole-HT used in HT OSP has the lowest volatility, compared to other OSP coatings tested.
? HT OSP has the highest decomposition temperature, compared to other OSP coatings tested.
? After five lead-free reflow cycles, HT OSP has only a 1% increase in oxygen content compared to a 7.5% increase for the industry standard OSP . Also HT OSP has almost no discoloration after five lead-free reflow cycles.
? HT OSP offers excellent solder reliability in terms of through-hole tests, wetting balance tests after more than three lead-free reflow cycles because of its unusual thermal resistance.
? HT OSP provides high reliability solder joints as evidenced by shear strength tests. n
For more information, email Shenliang Sun of Enthone Inc. at ssun@https://www.wendangku.net/doc/f711871374.html,.
References
1. Gi Xue, Quinpin Dai, and Shuangen Jiang, J. Am. Chem. Soc . 1988,110, 2393-2395.
2. Gi Xue, Junfeng Zhang, Applied Spectroscopy , Volume 45, Number 5, 1991.
3. G.P . Brown and S. Aftergut, Journal of Polymer Science: Part A , Volume 2, PP . 1839-1845 (1964).
4. Joint Industry Standard: Solderability Tests for Printed Boards, IPC/EIA J-STD-003A, September 2002.
5. Koji Saeki, “Next Generaration OSP for Lead-free Soldering, Mixed Metal Finish PWBs and BGA Substrate,” TPCA, 2005.
6. Lin Keh Wen, Michael Carano, “Quality and Reliability Test Methods Introduction of Organic Solderability Preservatives (OSP) for Lead-free Soldering and Results Comparison, TPCA Forum 2005.
7.Jian Fang-Fang, Tong Yu-Ping, Xiao Hai-Lian, Wang Qing-Xiang, Jiao Kui, “Structure and Thermal Properties of Transition Metals Imidazole Chloride.” Chinese Journal Structural Chemistry , Vol. 23, No.9, PP . 979-984 (2004).
8.E. K. Chang, etc. “Reflow Atmosphere Effects on An Organic Solderability Preservation (OSP) Coating,” Surface Mount
International (SMI), 1995.
Figure 5. Through-Hole solderability of HT OSP and industry standard OSP (LS
500A flux, SAC 305 solder).